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Creating a shared and static library with GCC

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NickSoft
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Joined: 13 Nov 2006
Posts: 22

Posted: Mon Jul 16, 2007 1:44 pm Post subject: Creating a shared and static library with GCC

The code for the library
helloworld.c:

Code:

#include <stdio.h>
void hello() {
printf("Hello world!\n");
printf("Creating a shared and static library with GCC howto by NickSoft Linux Cookbook (http://lcb.croler.net/)\n");
}

The header file
helloworld.h:

Code:

void hello();

Create object file:

Code:

gcc -c helloworld.c -o helloworld.o

Create static library

Code:

ar rcs libhello.a helloworld.o

Create shared library
gcc -shared -o libhello.so helloworld.o

A program using the library
main.c:

Code:

#include <stdio.h>
#include "helloworld.h"

int main(int argc, char* argv[]) {
hello();
return 0;
}

Linking against static library
Static library is archive of .o files, so we add them to gcc command line as we would add object files:

Code:

gcc -c main.c
gcc main.o libhello.a -o statically_linked

Note that all *.o files must be listed before static library (libhello.a)
Don't get confused by -static gcc option - it tells gcc to link statically against all libraries - that include libc.a and other system libraries. Linking all libraries statically will increase size of your executable with about 400kb (this may vary for different versions of gcc/linux distributions). The "-static" linking is used when you need to run the executable in different linux distributions without recompiling it.

Linking against shared library

Code:

LD_RUN_PATH=`pwd`
gcc main.c -o dynamically_linked -L. -lhello

or use -rpath linker option

Code:

gcc main.c -o dynamically_linked -Wl,-rpath,$PWD -L. -lhello

You can check if linking is successful with ldd:

Code:

ldd dynamically_linked
ldd statically_linked

You can see that statically linked executable is linked to other shared libraries, but it is statically linked to libhello.a

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